Separation of thiophenes from hydrocarbons by azeotropic distillation



Patented Apr. 13, 1948 2,439,777 SEPARATION-F TH IOPHENES FROM DROCARBONS BY AZEOTROPIC DISTIL- LATION George R. Lake and Josephine M. StriblemLong Beach, Calif., assignors to Union Oil Company of California, Los Angeles, Calif., a corporation of California No Drawing. Application September 18, 1944,

Serial No. 554,722 g This invention relates to a process of azeotropic distillation to prepare pure sulfur compounds from complex petroleum fractions which are diflicult to separate by ordinary fractional distillation dueto small difierences in boiling points existing between the hydrocarbons and the sulfur compounds contained in the fraction. More particularly the invention relates to a proc- 5 Claims. (Cl. 202-42) ess for separating pure. sulfur compounds from aromatic hydrocarbons, which sulfur compounds are virtuallyimpossible to separate from the arc matic hydrocarbons by ordinary fractional dis tillation because ofsmall differences in their boiling points.

The process for separating one hydrocarbon component from another hydrocarbon component of substantially the same boiling point contained in a complex hydrocarbon fraction by azeotropic distillation is well known. This process consists in distilling the hydrocarbon fraction in the Presence of an extraneous substance which has a preferential ailinity for one of the hydrocarbon components contained in the fraction thus causing tially the same boilingpoint found together in a disturbance of the vapor pressure equilibrium 1 that formerly existed in the fraction in such a manner that the vapor pressure or fugacity of at least one component of the fraction is changed sufficiently to permit its separation by controlled fractional distillation. Heretofore in such an azeotroping process there has been effected the separation of parafiins, naphthenes, olefins and aromatics from each other by the use of many varied types of azeotrope formers. However, it has been found in the work done in the azeotropic separation of such hydrocarbon components that the sulfur containing compounds norm-ally found in complex hydrocarbon fractions such as thiophenes, possess properties similar to the aromatic hydrocarbons contained in the fraction.

Because of this similarity between the'aromatic hydrocarbons and the sulfur compounds found in the presence of the aromatic hydrocarbons it has been found that in any application-of azeotropy to separate aromatics from non-aromatics contained in hydrocarbon fractions that sulfur compounds which may be contained therein boiling in the range of the aromatic hydrocarbons are found to act in a manner similar to said aromatic hydrocarbons and therefore are obtained with the aromatics in the final separation. For example, in the azeotropic distillation of a complex hydrocarbon fraction containing benzene and thio-- phene as well as naphthenic, paramnic. and ole-,- finic hydrocarbons in the presence of acetone as the azeotrope former, it is possible to take overhead azeotropes of acetone with the paramns, naphthenes, and oleilns in successive stages leaving as residue in the distillation the benzene and the thiophene. Prior to our invention there has been no simple method of separating the benzene from the thiophene in this residue. This has been found to be the case in those hydrocarbon fractions containing an aromatic hydrocarbonand a sulfur containing compound having a boiling point within 8 C. or 10 C. of each other.

It is an object of our invention to separate sulfur compounds from a complex hydrocarbon fracw tion in which they are'contained by a method which permits the recovery of the sulfur compound in comparatively pure form. ,More specifically it is an object of our invention to separate, by means of azeotropic distillation, sulfur compounds from aromatic compounds of essencertain petroleum fractions without destroying either component.

Other objects and advantages of our invention will become apparent to those skilled in the art as the description thereof proceeds.

According to our invention the separation of sulfur compounds from aromatic hydrocarbons is accomplished by azeotropic distillation wherein the azeotrope former consists of one member selected from the classes of compounds consisting of (l) the heterocyclic compounds contaimng at least one oxygen atom in the ring, (2) the heterocyclic compounds containing at least one-nitrogen atom in the ring, and (3) the nitriles.

The addition of an azeotrope former selected from these types of compounds which boils below but not more than 35 C. below the particular sulfur compound has the effect of forming a more volatile azeotrope with the aromatic hydrocarbon permitting its separation by distillation as an overhead product, leaving the sulfur containing compound as a residue,

In employing our process for the preparation of pure sulfur compounds we may use a complex petroleum fraction containing said sulfur compound as well as aromatic, naphthenic, paraffinic and olefinic hydrocarbons, wherein we employ an azeotrope former as disclosed herein, which azeotrope formers we have found will form azeotropes with the non-aromatics as well as the aromatic hydrocarbons in the fraction. These azeotropes are found to differ in boiling point sumciently to permit the concentration of paraflinic, naphthenic, olefinic or aromatic. hydrocarbons by carrying out the azeotropic distillation in successive stages wherein the dual purpose of purifying said components and of obtaining essentially pure sulto take overhead initially an azeotropic mixture" of dioxolane and paraf'nnic hydrocarbons and in a second stage an azeotropic mixture of dioxoiane and the naphthenic hydrocarbons, in a third stage an azeotropic mixture of dioxolane and the olefinic hydrocarbons and in a fourth stage an azeotropic mixture of dioxolane and benzene leaving. as bottoms in the distiilations essentially pure thiophene.

In a manner similar to the above, we are able to prepare essentially pure methyl thiophene from a petroleum fraction containing as well toluene and non-aromatic hydrocarbons by employing as an azeotrope former 1,3-dioxane,' plperidine, propionitrile, pyridine, orthe like, wherein the non-aromatic hydrocarbons together with the toluene are taken overhead as azeotropic mixtures with the azeotrope former leaving essentially pure methyl thiophene as azeotrope bottoms.

Similarly we may prepare pure dimethyl thiophenes from a complex petroleum fraction containing xylenes as well as non-aromatic hydrocarbons by employing such azeotrope formers as trioxane, pyrrole, or the like.

We may employ as azeotrope formers a saturated heterocyclic compound containing at least considerable quantities of sulfur compounds.

. The recovery of these compoundshas not in the past been deemed economically practical due tothe complexity and number of operations required to do so. In certainpetroleum stocks there are found to be considerable quantities of cyclic sulfur compounds suchas thiophene and its homologs which are virtually impossible to remove from such stocks without bringing about destruction of said sulfur compounds. For example, in the catalytic desulfurization of petroleum fractions the sulfur is removed from such cyclic compounds by conversion to hydrogen sulfide with the resultant conversion of the remaining portion of the molecule to a non-sulfur containing hydrocarbon of one type or another. In this manner for example methyl thiophene is converted to normal or i'sopentane upon catalytically removing the sulfur in the form of hydrogen sulfide. By the application of the process of our invention we are able to recover from such petroleum fractionsv as those described above pure cyclic sulfur compounds such as thiophene and its homologs. The difficulty of separating thiophene or its homologs from fractions in which they are contained by ordinary fractional distillation is readily appreciated upon examination of the boiling points of thiophene, methyl thiophene, dimethyl thiophene and the like, and the boiling point of benzene, toluene, xylene and higher boiling aromatics. Also as shown previously in conventional methods of areotropic distillation, the

l-oxygen atom in the ring such as for example furan,dioxane, dioxolane, trioxane, or the like, or a saturated heterocyclic compound containing at least l-nitrogenatom in the ring such as for example, pyrrole, pyrroledine, pyrazole, pyrazoline, pyridine, plperidine, or the like, or a nitrile such as for example acetonitrile, acrylonitrile, propionitrile, or the like. We have found that the selection of the particular azeotrope former depends upon the boiling point and composition of the particular hydrocarbon mixture to be treated, and upon the oxygenor nitrogen-carbon ratio in the azeotrope formers. For the separation of thiophene from benzene we may use as azeotrope formers such members of the above groups as dioxolane, pyrroledine, acetonitrile, and of this group dioxolane is the preferred azeotropic former. In like manner, for the separation of methyl thiophene from toluene, we may employ as azeotrope formers such members of the above types of compounds as 1,3-dioxane, plperidine, propionitrile, pyridine, and the like. Similarly in the separation of dimethyl thiophene from xylene, we may employ as azeotrope formers such compounds as trioxane, pyrrole-and the like. In general we have found that the heterocyclic compounds containing at least one oxygen or one sulfur compounds of the thiophene type have been found to function essentially as aromatic hydrocarbons and have therefore been prior to our invention impossible'to separate quantitatively from the aromatic hydrocarbon by azeotropic distillation. I

- The advantages of our invention are evident from the foregoing description thereof inasmuch as the pure sulfur compounds are at present very costly, although they are found in considerable quantities in petroleum. We are now able to obtain these sulfur compounds from petroleum by an economical and efllcient process of azeotropic distillation employing the motrope formers which we have found to be selective in forming azeotropes with aromatics in the presence of sulfur compounds such as thiophenes boiling in the range of said aromatic compounds.

Many methods of employing our process for separating sulfur containing compounds from hydrocarbon fractions in which they are contained will occur to those skilled in the art with out departing from the spirit and scope of the invention.

The type of operation employed will depend to a great extent upon the conditions enoountered and in the quality of the desired end products. We may for example employ the azeotrope formers as hereinbefore disclosed with comparatively wide boiling hydrocarbon fractions containing paraffinic, naphthenic, and oleflnic components as well as the aromatic and sulfur containing hydrocarbons. In such an application suflicient azeotrope former is required to form azeotropic mixtures with all of the hydrocarbons present which azeotropes may be taken overhead as individual concentrates, finishing the distillation at a temperature at least as-high as the boiling point of the azeotrope of the aromatic hydrocarbon with the azeotrope former, or as a broad mixture by continuing the distillation at a tempcrature sufiiciently high to vaporize the azeotropes of all of the hydrocarbons present in the a may employ a fraction boiling from 200 F. to

240 F. In the separation of methyl thiophene from a fraction such as this a sufilcient quantity of 1,3-dioxolane, piperidin'e, propionltrile, pyridine, or the like, is required to form azeotropes with the paraffin, naphthene, olefin and aromatic hydrocarbons contained in the fraction, which mixture is subjected to a controlled distillation to give said azeotropes as overhead products leaving essentially pure methyl thiophene as 'a distillation residue. V I

In another method of operation the above petroleum fraction may be azeotropically distilled in the presence of such azeotrope formers as acetone, methyl alcohol, methyl ethyl ketone, or the like, to remove the non-aromatic hydrocarbons from the fraction as azeotropic overhead,

the residue from said azeotrope distillation con sisting of a mixture of toluene and methyl thiophene which may in turn be subjected to distillation in the presence Of an azeotrope former according to our invention whichwiil yield as an azeotropic overhead a mixture of the aromatic hydrocarbon and the azeotrope former employed leaving the sulfur compound as a residue in the distillation. For example, in preparing pure thiophene from a hydrocarbon fraction containing parafiins, naphthenes, olefins, benzene, and thiophene we may. subject this mixture to an azeotropic distillation in the presence of an azeotrope former such as acetone, or methyl alcohol, taking overhead from said distillation azeotropic mixtures of said azeotrope former and the paraffins, naphthenes, and oleflns. inthe fraction leaving as residue inthis primary azeotropic distillation a mixture of benzene and thiophene. This mixture of benzene and thiophene may then be subjected to an azeotropic distillation in the presence of an azeotrope former as hereinbefore disclosed .such as dioxolane, pyrrolidine, acetonitrile, or

the like giving as an overhead product an azeotropic mixture of said azeotrope former with benzene leaving essentially pure thiophene as distillation residue.

'. The recovery of the azeotrope formers from The quantity and type of azeotrope former employed is dependent upon the composition and boiling range of the stock to be treated. It has been found that an azeotrope of benzene and dioxolane forv example consists of approximately 85% dioxolane and 15% benzene and boils at approximately 74 C. Similarly, we have found that the azeotrope of toluene, and 1,3-dioxane is of approximately the same composition. and when employing these azeotrope formers it is necessary that they be used in essentially the above ratio with the aromatic hydrocarbon contained in the stock. In the application of our process to {I80- We have found water wash- 1,3-dioxane.

tions containing parafllnic, naphthenic and oleflnic hydrocarbons as well as an aromatic hydrocarbon and a sulfur containing compound, it is necessary to employ a suflicient excess of v the azeotropic former to form azeotropes with the addition of .suihcient azeotrope former to re move the non-aromatics as well as the aromatics from a petroleum fraction may be based on these data.

Our preferred azeotrope for separating thiophene from a mixture containing benzene which mixture may or may not contain non-aromatic hydrocarbons is dioxolane. SimiIarly, our preferred azeotrope former in separating methyl thiophene from toluene which mixture may or may not contain non-aromatic hydrocarbons is In like manner, in the separation of dimethyl thiophene from xylenes, which mixture may or may not contain non-aromatic hydrocarbons our preferred azeotrope former is trioxane. Whereas, we prefer to employ these azeotrope formers in the anhydrous state, we have found that any or all of them may be satisfactorily employed in the presence of small amounts of water. The following example of our process will serve to show the economical manner in which relatively pure sulfur compounds may be obtained from petroleum fractions but is not intended to limit our invention in any way.

Example- A synthetic mixture containing 62% benzene and 38% thiophene was prepared which mixture is representative of benzene, thiophene concentrates which may be obtained from various petroleum stocks. volumes of this synthetic mixture was combined with 450 volumes of dioxolane and the resultant mixture subjected to a controlled fractional distillation in a column of approximately 30 theoretical plates. On the basis of the amount of benzene in the original fraction 350 volumes of dioxolane would represent the theoretical requirement for complete removal of the benzene if perfect fractionation were employed. For the purpose of our experiment a Zip/2% excess of dioxolane was employed.

The initial overhead product from the'above azeotropic distillation was obtained at vapor temperatures of74 C. to 745 C. and consisted of 60.7% of the original benzene-thiophene mixture. The azeotropic .mixture consisted of 15.3% of oil and 84.7% of dioxolane azeotrope former. The oil, after removal of the azeotrope formerby water washing analyzed better than 99% benzene. The transitional fraction representing 1.4% of the dioxolane free oil was taken overhead attemperatures'of 755 C. to 76.7 C. and. contained on dioxolane free bases 72.8% benzene and 28.2% thiophene. This transition fraction was found to contain 98% dioxolane and 2% of hydrocarbon oil of the above composition. In commercial operation it might be desirable to stop the distillation at this point which would contaminate the thiophene with only about 1% of benzene which would not be objectionable in most cases. The bottoms from the distillation after removal of excess dioxolane by water washing represented 38% of the original benzene-thiophene mixture and consisted of thiophene of better than purity.

It is seen from this example that by the process of our invention, it is not only possible but;

economically feasible to obtain pure thiophene from high sulfur hydrocarbon fractions containing thiophene and benzene which benzene has heretofore made the separation of said thiophene virtually impossible.

1. A process for the separation of a sulfur com pound of the group consisting of thiophene and its homologs from a complex hydrocarbon fraction containing paraiflns, naphthenes, olefina.

and aromatics as well as said sulfur compound all of which hydrocarbons ordinarily distill from said fraction in the same temperature range as said sulfur compound distills therefrom which comprises azeotropically distilling said complex hydrocarbon fraction in the presence of a sumcient amount of an azeotrope former selected from the group consisting of the heterocyclic compounds containing at least one oxygen atom in the ring, the heterocyclic compounds containing at least one nitrogen atom in the ring,

and the nitriles to vaporize said paraiilns, naphthenes; olefins and aromatics, continuing the distillation at a temperature sufiiciently high to vaporize the azeotropes of all of said hydrocarbons with said azeotrope former until said parafilns, naphthenes, olefins, and aromatics are completely vaporized together with said azeotrope former thereby leaving said sulfur compound in the residue, said azeotrope former boiling below but not more than 35 C. below said sulfur compound. v

2. A process for the separation of benzene and thiophene which comprises azeotropically distilling the benzene and thiophene mixture in the presence of dioxolane as an azeotrope former wherein the benzene is removed from the mixture as an azeotropic distillate with dioxolane leaving a distillation residue enriched in thiophene.

3. A process for the separation of toluene and methyl thiophene which comprises amotropically distilling the toluene and methyl thiophene' mixture in the presence of propionitrile as an azeotrope former wherein the toluene is removed from the mixture as an azeotropic distillate with propionitrile leaving methyl thiophene in the residue.

4. A process for the separation of xylene and dimethylthlophene which comprises azeotropically distilling the xylene and dimethyl thiophene mixture in the presence of pyrrole as an azeotrope former wherein the xylene is removed from the mixture as an azeotropic distillate with pyrrole leaving dimethyl thiophene in the residue.

5. A process for the separation of sulfur compounds of the group consisting of thiophene and its homologs and aromatic hydrocarbons commonly found together in petroleum distillates, which sulfur compounds are diflicult to separate from said aromatic hydrocarbons by ordinary fractional distillation due to the small differences in their boiling points, which process comprises azeotropically distilling the mixture of sulfur compounds and aromatic hydrocarbons in the presence of an amount of azeotrope former sufficient to form azeotropes with all of the aromatic hydrocarbons present and-- continuing the distillation at an overhead temperature at least as high as the boiling point of the azeotrope of the aromatic hydrocarbon with the azeotrope former until substantially all of said aromatic hydrocarbons are vaporized together with said azeotrope former thereby leaving said sulfur compounds in the residue, said azeotrope former boiling below, but not more than 35 C.

below said sulfur compounds and said azeotrope former being selected from the class of compounds consisting of the heterocyclic compounds containing at least one oxygen atom in the ring,

the heterocyclic compounds containing at least one nitrogen atom in the ring and the nitriles.

GEORGE R. LAKE. JOSEPHINE M. STRIBLEY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS OTHER REFERENCES Mair et al., Bureau of Standards Journal of Research, vol. 27, pages 44-50, 56, 57. 

